Electrochemical fluorination is a process whereby the passage of an electrolytic current is made to incorporate fluorine into a substrate by addition or substitution. This process can be used to produce perfluorocarbons, a class of compounds or substances known for their outstanding chemical, electrical, and thermal stability. It can also be used to produce materials which are reactive and useful as chemical intermediates; these products include hexafluoroacetone, perfluoroacyl fluorides, and perfluoroesters. Other products include sulfur hexafluoride and carbonyl fluoride.
The passage of this electrolytic current (electrolysis) is effected through an electrolyte which usually is liquid hydrogen fluoride or hydrogen fluoride complexed with a current-conducting additive such as an alkali metal fluoride. This current is passed between a cathode and a porous anode contained in an electrolysis cell; hydrogen gas is evolved at the cathode; and fluorine is generated at the anode. The fluorine reacts with the material to be fluorinated while it is within the pores of the anode.
It is known that production rate is contained in large part by the density of the current within the electrolyte solution, which for a normal one-cell unit, such as shown in U.S. Pat. No. 3,882,001, is about 2.5 to 3.0 K amp/m.sup.2. The disclosure of the patent is incorporated by this reference. Increasing the current density with a corresponding increase in feed rate increases production rates, but it also increases temperature within the cell frequently beyond the capabilities of the heat-removing apparatus of the cell. Mass transfer is an important part of heat transfer, and, in the usual electrochemical fluorination apparatus, this mass transfer is carried out by thermal siphon and by hydrogen lift.
Although fairly efficient, the process is hazardous because of the toxic and corrosive nature of hydrogen fluoride. This makes maintenance difficult during cell repair. With certain cell configurations, feed lines become plugged with salts when the feed is passed from the bottom of the cell through the electrolyte into the anode area. When the feed is passed through the top of the cell down into the anode area, plugging at least at the anode is avoided.
An object of this invention is to provide an electrochemical object. Another operation of this invention is to provide an electrochemical cell structure. Still another object of this invention is to provide a feed introduction means permitting improved design and multiple electrodes containing cell structure. A further object of this invention is to provide such a multiple electrode electrochemical cell structure. A further object of this invention is to provide an optimum designed electrochemical cell structure which can be designed disregarding problems due to evolution of heat. A further object of the invention is to provide a structure for an electrochemical cell such that plugging of the cell is entirely avoided.
Other aspects, concepts, objects, and the several advantages of the invention are apparent from a study of this disclosure, the drawing, and the appended claims.
According to the present invention, there is provided a process for conducting an electrochemical reaction in an electrochemical reaction zone or cell which comprises circulating at least a portion of the electrolyte in said zone from said zone, adding feed to cooling said electrolyte, and then returning at least a portion of the electrolyte containing said feed to said reaction zone.
Still according to the invention, there is provided an electrochemical cell having multiple electrodes therein and means associated with said cell for cooling the electrolyte externally of said cell.
Further according to the invention, there is provided a process for conducting an electrochemical reaction in an electrochemical reaction zone or cell having multiple electrodes therein which comprises circulating at least a portion of the electrolyte in said zone from said zone to a cooling zone, in said cooling zone cooling said electrolyte, and then returning at least a portion of the thus-cooled electrolyte to said reaction zone.
Still further according to the invention feed added to the circulating electrolyte is returned to the area at the bottom of the anodes in the cell for passage upwardly through the anodes.
In an embodiment according to the invention there is provided, see FIG. 1, a feed disengaging area at the bottom of the anode to ensure that feed will disengage from the electrolyte and pass upwardly through the anode area while the electrolyte, upon disengagement of the feed, is passed through the space or area between an anode and a cathode.
In a specific embodiment of the invention, there is provided an electrochemical fluorination operation of a cell structure according to an embodiment of the invention.
Advantages of the process of the invention are as follow:
1. The feed is introduced through the bottom of the multi-cell unit. Thus the top of the assembly is less cluttered as are other configurations, making maintenance operations simpler and safer. This can be and in the now preferred embodiment is done by introducing the feed with the cooled, recycled electrolyte as described below. These advantages are made possible by the introduction of the feed into the electrolyte.
2. The electrolyte is circulated and cooled externally to the cell using a pump and external heat exchanger. Such pumping permits faster flow of the electrolyte than can be obtained with the usual thermal siphon and hydrogen gas lift. This higher flow will permit operation at a higher current density at the same voltage, or at the same current density with a lower voltage. Thus, one can obtain higher productivity or lower electrical energy consumption. The invention permits a simpler, more compact cell design since no provision need be made for internal heat exchange surface or down-comer tubes. It also permits simple filtering of the electrolyte to remove the sludge that accumulates in cell operation.
3. It is simpler to accurately split a large volume stream of electrolyte containing dispersed feed into several streams, for feeding individual streams for such feeding. The small orifices ordinarily needed to split just the feed stream when not added to the electrolyte as is done in the present invention, are prone to plug and corrode as the recycle feed frequently contains considerable HF and, on occasion, some heavy material.
The single cell data used herein to illustrate the invention are based on the process described by R. B. MacMullin et al., J. Electrochem. Soc., Vol. 118, No. 10, 1582 (1971), and U.S. Pat. No. 3,711,396. Porous electrodes of a particular kind for use in electrochemical processes are described and claimed in U.S. Pat. No. 3,558,450. These disclosures are incorporated herein by reference.